Hot water heating systems are conventionally provided in domestic residences, public buildings, schools, small apartment buildings and the like, with a series of pipes and radiators in each room through which hot water is circulated. The water is heated by means of a solid fuel fired boiler located in the basement of the building, the water being circulated continuously through the system and through a heat exchanger associated with the boiler. With the ever increasing costs of heating fuels, it is desirable that any such boiler be used to its maximum efficiency in heat transfer, and that it be capable of burning a wide variety of different fuels, including refuse, for this purpose. It is also desirable that full control be exercised over the temperature of the water issuing from the heat exchanger associated with the boiler, to avoid overheating and wastes of fuel capacity.